1. Field of the Invention
The present invention relates to a copier, printer, facsimile apparatus or similar image forming apparatus operable in a duplex print mode for printing images on both sides of a sheet or recording medium.
2. Description of the Related Art
It is a common practice with an image forming apparatus operable in a duplex print mode to transfer a toner image from an image carrier to one surface of a sheet, fix the toner image, turn the sheet via, e.g., a turn path, and again feed the sheet to form another toner image on the other side of the sheet. The problem with this type of apparatus is that the sheet cannot be reliably conveyed due to the switching of the sheet conveying direction and the curl of the sheet ascribable to the fixation of the toner image on one side of the sheet.
In light of the above, Japanese Patent Laid-Open Publication No. 1-209470 discloses an image forming apparatus including a first and a second image carrier for transferring toner images to both sides of a sheet and then fixing them at the same time. More specifically, in the apparatus taught in this document, a first image formed on a photoconductive element is transferred to an image transfer belt by first image transferring means. Subsequently, a second toner image formed on the photoconductive element is transferred to one side of a sheet. Thereafter, the first image is transferred from the belt to the other side of the sheet by second image transferring means. The sheet carrying the toner images on both sides thereof is conveyed to a fixing unit.
However, the procedure taught in the above document is not practicable without causing the image transfer belt to make two turns. More specifically, the second image begins to be formed only after the image transfer belt has made one full turn, resulting in lower productivity in the duplex print mode. This is particularly true when full-color images are formed on both sides of a sheet.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication No. 8-160703.